There has been conventionally known an active noise reducing device that actively reduces noise by outputting a cancel sound for cancelling out the noise from a cancel sound source through the use of a reference signal correlating with the noise and an error signal based on a residual sound in which the noise and the cancel sound interfere with each other in a predetermined space (for example, refer to PTL 1). The active noise reducing device generates a cancel signal to output the cancel sound using an adaptive filter such that a sum of square of the error signal becomes minimum.